In recent years, with the increase in energy density due to miniaturization and high performance of electronic devices, the amount of heat generation per unit volume tends to increase, so that high thermal conductivity is demanded for insulating materials constituting such electronic devices. Epoxy resin is widely used as an insulating material from the viewpoint of high dielectric strength and ease of molding. As a method of increasing the thermal conductivity of an epoxy resin, for example, Japanese Patent Application Laid-Open (JP-A) No. H11-323162 describes that it is effective to utilize a liquid crystalline epoxy resin obtained by polymerizing a resin composition containing a monomer having a highly oriented mesogenic group.
Further, in order to increase the thermal conductivity of an epoxy resin, a method of adding a thermally conductive filler having a high thermal conductivity and an insulating property to a resin to be composited is generally used. Examples of the thermally conductive filler include boron nitride particles and alumina particles.
The shape of the thermally conductive filler may be spherical, ellipsoidal, scale-like, plate-like, and the like, and is selected according to the application. As a method of increasing the thermal conductivity in the thickness direction when the filler is formed into a sheet-shaped cured product, for example, Japanese National-Phase Publication (JP-A) No. 2008-510878 describes that it is effective to incorporate a specific spherical boron nitride aggregate having an average aspect ratio of less than 2 into a polymer matrix.
As a method of increasing the thermal conductivity of a composite of a resin and a thermally conductive filler, for example, Japanese National-Phase Publication (JP-A) No. 2011-503241 describes that it is effective to form an ordered resin shell around a thermally conductive filler so that the resin molecules align perpendicular to the surface of the thermally conductive filler, and furthermore, to overlap the ordered resin shells so that a continuous path is formed by the ordered resin shells.